2 Hartke 410TPs or Hartke 4200

I currently run a hartke 410TP cabinet through a Hartke 3500 head. I am constantly being drowned out by the guitarists in my band. I want to know if I should add another 410TP (8ohm), or just run one 4200 cabinet(4ohm). My head rates something like 240 at 8ohms and 350W at 4ohms. I am not very knowledgeable when it comes to this aspect of bass playing. A sound guy told me that I would get more power and sound adding another 8ohm cabinet and that it would actually drop my ohms down to 4 with two 8ohm cabinets. Someone from Hartke told me to go with the 4ohm cabinet and that adding another 8ohm cabinet would just be a tonal change not volume. I don't really have any idea. I need advice.

Two 8 ohm cabs = 4 ohm load. The amp will put out 350w at 4 ohms, like you said. More important than the wattage, more speakers will also move a lot more air, increasing volume.

I'd recommend getting an 8 ohm 410XL, if you want to add another 410. You could find a nice one used, and they're a lot more common than the 4200. The XL uses different speakers than the TP, so there would probably be a difference in tone, but it would be for the better. I haven't played a 4200, so I can't comment on that.

Ideal "un-drownable" Hartke setup is 4.5XL + 215XL. I think you should think of adding a 15" (even the Transporter series) to your setup in order to gain loudness. Hartke's alluminium 15"s are not muddy at all, they're fast to move so you don't lose punch and they produce BIG volume. Good luck!